**Core Concept**
Enteric fever, also known as typhoid fever, is a systemic infection caused by two main pathogens: *Salmonella Typhi* and *Salmonella Paratyphi*. This disease primarily affects the gastrointestinal tract and can lead to complications such as intestinal perforation and bacteremia.

**Why the Correct Answer is Right**
The correct answer is *Salmonella Typhi* and *Salmonella Paratyphi*. These pathogens are transmitted through contaminated food and water, and the bacteria invade the intestinal mucosa, causing inflammation and fever. The bacteria then enter the bloodstream and can cause systemic infection. The key mechanism involves the invasion of the intestinal epithelial cells by the bacteria, which is facilitated by the presence of type III secretion systems.

**Why Each Wrong Option is Incorrect**
**Option A:** *Shigella dysenteriae* is a cause of dysentery, not enteric fever. While it also causes gastrointestinal infection, the clinical presentation and pathophysiology differ from enteric fever.
**Option B:** *Campylobacter jejuni* is a common cause of gastroenteritis, but it is not typically associated with systemic infection or the characteristic fever of enteric fever.
**Option C:** *Escherichia coli* is a common intestinal inhabitant and can cause urinary tract infections, but it is not a primary cause of enteric fever.

**Clinical Pearl / High-Yield Fact**
The presence of abdominal tenderness and guarding in a patient with fever should raise suspicion for enteric fever, especially if the patient has a history of consuming contaminated food or water.

**Correct Answer: C. Salmonella Typhi and Salmonella Paratyphi.**